Asymmetric birdcage coil for a magnetic resonance imaging (MRI)

ABSTRACT

A birdcage coil includes: (a) a pair of conductive end rings, each having a generally domed shape in axial cross section; (b) a plurality of conductive, elongated rungs extending between the pair of conductive end rings in an axial direction; and (c) an LC delay circuit incorporated into the pair of rings and the plurality of elongated rungs, where the LC delay circuit includes a plurality of capacitive elements and a plurality of inductive elements. In the present invention circumferential spacing between adjacent elongated rungs is varied to improve homogeneity of the volume excitation. Alternatively, or in addition, LC circuit capacitance and/or inductance values are varied to improve homogeneity of the volume excitation.

CROSS REFERENCE TO RELATED APPLICATIONS

The current application is a continuation application of U.S. patentapplication Ser. No. 16/437,234 filed Jun. 11, 2019 (issuing as U.S.Pat. No. 10,884,079 on Jan. 5, 2021), and claims priority to U.S.Provisional Application, Ser. No. 62/683,252 filed Jun. 11, 2018, theentire disclosures of which are incorporated herein by reference.

BACKGROUND

Magnetic Resonance Imaging (MRI) employs a strong magnetic field, B0,that is used to polarize the spin magnetization in a patient's body. Thespin magnetization that is most often used in MRI arises from the nucleiof hydrogen atoms within the body. Although the highest concentration ofhydrogen atoms within the body is found in water molecules, othercompounds found in the body (e.g. lipids, glucose, etc.) are present insufficient concentration to provide a detectable MR spin magnetization.MRI can be performed on a number of nuclei such as hydrogen-1 (referredto as proton), helium-3, lithium-7, carbon-13, fluorine-19, oxygen-17,sodium-23, phosphorus-31 and xenon-129. In most MRI applications,hydrogen-1 can be preferred due to its high gyromagnetic ratio andabundance in most tissues in the body, which can translate into highsignal-to-noise ratio (“SNR”).

When spin ½ nuclei of a patient's body are introduced into thepolarizing magnetic field, the spin magnetization of the nuclei align inone of two states: with the magnetic field, or against the magneticfield. These two states occupy slightly different energy levels in aquantum mechanical system. By convention, the lowest energy level iscalled the ground state. It should be noted that the population ofnuclear spins in the ground state is slightly higher than that of thehigher energy state, resulting in a net magnetization of the macroscopicgroup of nuclei.

The energy difference between the two energy levels is directlyproportional to the strength of the polarizing magnetic field. Thus, asthe strength of the magnetic field is increased, the energy differencebetween the two states increases. The energy differences associated withtypical MRI systems correspond to electromagnetic waves in theradiofrequency range. The specific frequency associated with thedifference is called the Larmor frequency (typically given in MHz). Theconstant of proportionality that defines the relationship between thepolarizing field (typically given in Tesla) and the Larmor frequency isa natural constant called the gyromagnetic ratio. This constant isunique for each MR active element. For Magnetic Resonance Imagingsystems used in medicine, polarizing magnetic field fields are typicallybetween 0.5 and 3.0 Tesla. For hydrogen atoms, these polarizing magneticfield strengths result in Larmor frequencies between 21.3 and 127.8 MHz.For xenon-129, these polarizing magnetic field strengths result inLarmor frequencies between 5.89 and 35.33 MHz. For xenon-129 a dedicatedtransmit coil is required with 35.3 MHz resonance at 3 T.

If the nuclear spin system immersed in a polarizing magnetic field issubjected to a rotating magnetic field at the Larmor frequency, 131, thespin system will absorb energy and the distribution of nuclear spins inthe two energy states will be disturbed. The duration of the rotatingmagnetic field used to change the distribution of nuclear spins in thetwo energy states is typically limited, and applied with a strengthsufficient to nutate the net spin magnetization from the longitudinalaxis (i.e. parallel with the applied polarizing magnetic field) to thetransverse plane (i.e. perpendicular to the applied polarizing field).The term “RF pulse” is conventionally used to describe this processsince nutation is accomplished with a rotating magnetic field in theradiofrequency range and having a finite duration.

With time, the energy will be emitted by the spin system in a fashionthat can be detected with a sensitive pickup coil. The absorption andre-emission of an RF signal is key to the formation of an MR image. Thisphenomenon is typically called “resonance”.

When an MR signal is created, the frequency of the signal is preciselyproportional to the strength of the magnetic field experienced by thenuclear spins. If all of the spins in a patient's body are in anidentical magnetic field, then all the spins will resonate with the samefrequency. Even though signals come from many different portions of thebody, the MR imaging system has no way to distinguish one signal fromanother.

In order to provide spatial encoding of the MR signals (and hence enablethe formation of an image), it is useful to create a transientinhomogeneity in the magnetic field. In typical MRI imaging systems thisis accomplished with magnetic field gradient coils. Gradient coilstypically are designed to create a magnetic field whose strength variesin a linear fashion over a selected volume within the magnet. Gradientcoil sets are typically constructed to permit gradient fields to becreated in three orthogonal directions within the bore of the magnet.Typical gradient coils driven by typical gradient amplifiers cangenerate a magnetic field gradient of 20 mT/m in less than 1 ms, andmaintain that gradient with high fidelity for an extended period limitedonly by the heat dissipation of the gradient coils and amplifier.

A typical imaging system creates an image by employing a sequence of RFand magnetic field gradient pulses to establish a detectable MR signalin a selected plane. This signal is then spatially encoded usingmagnetic field gradient pulses to impart phase and frequency shifts tothe MR signal which reveal the location of the signal source within thebore of the magnet. By selecting pulse sequence repetition times (TR),echo times (TE) and other pulse sequence parameters, the operator cantune the pulse sequence to be sensitive to a variety of intrinsic MRparameters found in the tissue of the patient (e.g. longitudinalrelaxation time, T1, Transverse relaxation time, T2, and the like). Manypulse sequences are known to those skilled in the state of the art.These pulse sequences can collect data in two or three dimensions. Theycan also collect data in Cartesian, radial or spiral frameworks.

Hyperpolarized ¹²⁹Xe imaging is increasingly viewed as a viable tool forassessing lung structure and function in patients¹. Additionally,HP¹²⁹Xe is moderately soluble in tissues and possesses a large (>200ppm) in-vivo chemical shift range, making dissolved-phase ¹²⁹Xe MRuniquely sensitive to regional gas-exchange dynamics²⁻⁴. Selectiveexcitation of dissolved ¹²⁹Xe requires that large flip angle andfrequency selective RF pulses be applied homogeneously across the entirethorax to avoid unintentionally exciting the gas-phase ¹²⁹Xe. Ingeneral, the flip angle of an RF pulse increases with the duration ofthe pulse. With ¹²⁹Xe MRI, however, a pulse duration that is longer thanthe gas exchange time loses its selectivity. Thus, there is a need forhigh B1 fields during excitation of ¹²⁹Xe in the dissolved state.Furthermore, ¹²⁹Xe body coils that fail to provide sufficiently uniformB1 fields will provide a spatially dependent B1 field. Since the flipangle of the RF pulse is an important contributor to the quantitativemeasurements of gas exchange, a homogeneous ¹²⁹Xe body coil is desired.

Body coils in the form of birdcage coils are traditionally constructedusing a cylindrical birdcage design with equidistant rung-spacingcentered in the magnet's RF shield. Due to the restricted spaceavailable for placing a ¹²⁹Xe body coil in a general-purpose MRI system,asymmetrical ¹²⁹Xe birdcage coils have been developed to fit within themagnet bore and yet account for the presence of the patient table. Suchasymmetrical coils have a generally flat bottom for riding on top of thepatient table⁵⁻⁷, producing a generally domed shape in cross-section(generally flat bottom to account for patient table but a cylindricalside and top to match the curvature of the magnet bore). However, the B1homogeneity of theses designs was evaluated with the coil unloaded andonly within the central slice locations. Thus, it remains unclear ifthese designs are suitable for truly quantitative dissolved-phase ¹²⁹XeMRI.

Thus, there is a need to develop such asymmetrical birdcage coil designsfor homogeneous volume excitation for ¹²⁹Xe imaging (and nuclei).

SUMMARY

To develop a suitable large and homogeneous ¹²⁹ Xe 3 T birdcage coil,the volume excitation performance of multiple coil designs was analyzedwith electromagnetic simulations. The influence of the shield size andposition of the coils were analyzed in detail. Results of the analysiswere used to confirm exemplary optimized designs for homogeneous volumeexcitation necessary for ¹²⁹ Xe imaging.

As a result of such analysis, a first aspect of the current disclosureis to provide a birdcage coil for a magnetic resonance imaging (MRI)system that includes: (a) a pair of conductive end rings, each having agenerally domed shape in axial cross section; (b) a plurality ofconductive, elongated rungs extending between the pair of conductive endrings in an axial direction; and (c) an LC delay circuit incorporatedinto the pair of rings and the plurality of elongated rungs, where theLC delay circuit includes a plurality of capacitive elements and aplurality of inductive elements; where circumferential spacing betweenadjacent elongated rungs is varied to improve homogeneity of the volumeexcitation.

In a detailed embodiment of this first aspect, the length of theelongated rungs is varied so that the area between adjacent elongatedrungs is substantially the same. In a further detailed embodiment, oneor both of the pair of end rings are non-planar to account for thevaried lengths of the elongated rungs.

In another detailed embodiment of this first aspect, the capacitiveelements in the LC delay circuit are varied. In a further detailedembodiment, the capacitive elements in the LC delay circuit are variedto maintain a steady speed of delay about a circumference of thebirdcage coil.

In another detailed embodiment of this first aspect, the variedcircumferential spacing between adjacent elongated rungs is symmetricalbetween the left side and right side of the birdcage coil (where thebottom is the generally flat portion). Alternatively, or in addition,the varied circumferential spacing is wider on a bottom half and closeron a top half.

In another detailed embodiment of this first aspect, the birdcage coilis a high-pass birdcage coil, or the birdcage coil is a low-passbirdcage coil, or the birdcage coil is a band-pass coil. Alternatively,or in addition, the birdcage coil includes sixteen of the conductive,elongated rungs extending between the pair of conductive end rings in anaxial direction.

A second aspect of the current disclosure is to provide a birdcage coilfor a magnetic resonance imaging (MRI) system that includes: (a) a pairof conductive end rings, each having a generally domed shape in axialcross section; (b) a plurality of conductive, elongated rungs extendingbetween the pair of conductive end rings in an axial direction; and (c)an LC delay circuit incorporated into the pair of rings and theplurality of elongated rungs, where the LC delay circuit includes aplurality of capacitive elements and a plurality of inductive elements;where at the capacitive elements and/or inductive elements are varied toprovide a uniform rotational velocity of the magnetic field about acircumference of the birdcage coil. In a more detailed embodiment, thegenerally domed shaped end rings have a generally flat bottom, agenerally rounded top, a left side and a right side; and the variedcapacitive elements and/or inductive elements are symmetrical betweenthe left side and right side. Alternatively, or in addition, the variedcapacitive/inductive elements involves varied inductive elements. In amore detailed embodiment, the varied inductive elements involves varieddimensions of the elongated rungs. In a more detailed embodiment, thevaried dimensions of the elongated rungs involve varied lengths of theelongated rungs and/or varied diameters of the elongated rungs.

A third aspect of the current disclosure is to provide a birdcage coilfor a magnetic resonance imaging (MRI) system that includes: a pair ofconductive end rings, each having at least one asymmetric dimension inaxial cross section; a plurality of conductive, elongated rungsextending between the pair of conductive end rings in an axialdirection; and an LC delay circuit incorporated into the pair of ringsand the plurality of elongated rungs, where the LC delay circuitincludes a plurality of capacitive elements and a plurality of inductiveelements; where circumferential spacing between adjacent elongated rungsis varied to improve homogeneity of the volume excitation.

In a detailed embodiment of the third aspect, the length of theelongated rungs is varied so that the area between adjacent elongatedrungs is substantially the same. In a further detailed embodiment, thepair of end rings are non-planar to account for the varied lengths ofthe elongated rungs.

In another detailed embodiment of the third aspect, the capacitiveelements in the LC delay circuit are varied. In a further detailedembodiment, the capacitive elements in the LC delay circuit are variedto maintain a steady speed of delay about a circumference of thebirdcage coil.

These and other aspects and objects of the current disclosure will beapparent from the following description, the appended claims and theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram representation of an MRI system environmentfor use with embodiments of the current disclosure;

FIG. 2 is a perspective view of an exemplary birdcage coil according toan embodiment of the current disclosure;

FIG. 3 is an end view of the exemplary birdcage coil of FIG. 2;

FIG. 4 is a side view of the exemplary birdcage coil of FIG. 2;

FIG. 5 is a bottom view of the exemplary birdcage coil of FIG. 2;

FIG. 6 is a circuit capacitance diagram representation of the birdcagecoil of FIG. 2;

FIG. 7 is a representation of the simulation set-up using an ellipticalphantom for developing birdcage coil designs disclosed herein;

FIG. 8 is a representation of three initial analyzed coil designs in theanalysis;

FIG. 9 is a representation of the coil positions inside the RF shieldfor the analyzed coil designs of FIG. 8;

FIG. 10 shows the B₁ ⁺ field distribution for the central axial slicewith the coils of FIG. 8 centered inside the RF shield.

FIG. 11 shows the percentage of B₁ ⁺ within +/−5% from mean and largerthan +/−5% from mean for the coils of FIG. 8 centered inside the RFshield;

FIG. 12 shows the B₁ ⁺ volume histogram split into 100 bins for thecoils of FIG. 8 centered inside the RF shield;

FIG. 13 shows the B₁ ⁺ field distribution for the central axial slicewith the coils of FIG. 8 adjusted to the patient table;

FIG. 14 shows the percentage of B₁ ⁺ within +/−5% from mean and largerthan +/−5% from mean for the coils of FIG. 8 adjusted for the patienttable;

FIG. 15 shows the B₁ ⁺ volume histogram split into 100 bins for thecoils of FIG. 8 adjusted for the patient table;

FIG. 16 is a representation of two exemplary improved (i.e.,“optimized”) coil designs according to the current disclosure;

FIG. 17 shows the B₁ ⁺ field distribution for the central axial slicewith the exemplary coils of FIG. 16 adjusted to the patient table;

FIG. 18 shows the percentage of B₁ ⁺ within +/−5% from mean and largerthan +/−5% from mean for the exemplary coils of FIG. 16 adjusted for thepatient table; and

FIG. 19 shows the B₁ ⁺ volume histogram split into 100 bins for theexemplary coils of FIG. 16 adjusted for the patient table.

DETAILED DESCRIPTION

MR imaging of internal body tissues may be used for numerous medicalprocedures, including diagnosis and surgery. In general terms, MRimaging starts by placing a subject in a relatively uniform, staticmagnetic field. The static magnetic field causes MR-active nuclei spinsto align and precess about the general direction of the magnetic field.Radio frequency (RF) magnetic field pulses are then superimposed on thestatic magnetic field to cause some of the aligned spins to alternatebetween a temporary high-energy nonaligned state and the aligned state,thereby inducing an RF response signal, called the MR echo or MRresponse signal. It is known that different tissues in the subjectproduce different MR response signals, and this property can be used tocreate contrast in an MR image. An RF receiver detects the duration,strength, and source location of the MR response signals, and such dataare then processed to generate tomographic or three-dimensional images.

FIG. 1 shows an exemplary MRI system 100 in or for which MR imaging inaccordance with the present disclosure may be implemented. Theillustrated MRI system 100 comprises an MRI magnet assembly 102. Sincethe components and operation of the MRI scanner are well-known in theart, only some basic components helpful in the understanding of thesystem 100 and its operation will be described herein.

The MRI magnet assembly 102 typically comprises a cylindricalsuperconducting magnet 104, which generates a static magnetic fieldwithin a bore 105 of the superconducting magnet 104. The superconductingmagnet 104 generates a substantially homogeneous magnetic field withinan imaging region 116 inside the magnet bore 105. The superconductingmagnet 104 may be enclosed in a magnet housing 106. A support table 108,upon which a patient 110 lies, is disposed within the magnet bore 105. Aregion of interest 118 within the patient 110 may be identified andpositioned within the imaging region 116 of the MRI magnet assembly 102.

A set of cylindrical magnetic field gradient coils 112 may also beprovided within the magnet bore 105. The gradient coils 112 alsosurround the patient 110. The gradient coils 112 can generate magneticfield gradients of predetermined magnitudes, at predetermined times, andin three mutually orthogonal directions within the magnet bore 105. Withthe field gradients, different spatial locations can be associated withdifferent precession frequencies, thereby giving an MR image its spatialresolution. An RF transmitter coil 114 surrounds the imaging region 116and the region of interest 118. The RF transmitter coil 114 emits RFenergy in the form of a rotating magnetic field into the imaging region116, including into the region of interest 118.

The RF transmitter coil 114 can also receive MR response signals emittedfrom the region of interest 118. The MR response signals are amplified,conditioned and digitized into raw data using an image processing system120, as is known by those of ordinary skill in the art. The imageprocessing system 120 further processes the raw data using knowncomputational methods, including fast Fourier transform (FFT), into anarray of image data. The image data may then be displayed on a monitor122, such as a computer CRT, LCD display or other suitable display.

Due to the size required for a ¹²⁹Xe MRI body coil, asymmetricalbirdcage coils have been developed that allow the ¹²⁹Xe coil to fitwithin the bore and yet account for the presence of the patient table.Such asymmetrical coils have a generally flat bottom for riding on topof the patient table⁵⁻⁷, producing a generally domed shape incross-section (generally flat bottom to account for patient table but acylindrical side and top to match the curvature of the magnet bore). Theasymmetrical domed shape in cross-section allows the birdcage coils tofit snugly within the bore 105 opening over the support table 108.

FIGS. 2-5 provide various views of an example asymmetrical birdcage coil200 according the current disclosure. The asymmetrical birdcage coil 200has a pair of conductive end rings 202, each having a generally domedshape in axial cross section (as best seen in FIG. 3), and a pluralityof conductive, elongated rungs 204, extending between the conductive endrings 202. While the coil 200 in FIGS. 2-5 is a high-pass birdcage, itis within the scope of the current disclosure that the coil may be inthe form of a low-pass birdcage or a band-pass birdcage.

The example birdcage coil 200 of FIGS. 2-5 has a top portion 206 and abottom portion 208 that are coupled together to form the enclosing coilstructure. A surface 210 is provided on the interior portion of thebottom portion 208 for the patient to lie upon. Handles 212 are providedto allow ease in separating, handling and coupling together the top andbottom portions 206, 208.

The birdcage coil 200 also includes an LC delay circuit incorporatedinto the coil as is well known to those of ordinary skill. The LC delaycircuit will include a plurality of capacitive components and aplurality of inductive elements. In the embodiment shown in FIG. 2-5circumferential spacing between adjacent elongated rungs 204 is variedto improve homogeneity of the volume excitation.

The specific measured circumferential spacing between elongated rungs204 of the embodiment shown in FIGS. 2-5 (and the associated inductanceas a result of such spacing) is provided in the following Table 1 and 2:

TABLE 1 Top-Portion Birdcage End-ring Lengths Length Inductance FrontBack Front Back 1 54 mm 54 mm 21 nH 21 nH 2 78 mm 78 mm 36 nH 36 nH 3 81mm 81 mm 38 nH 38 nH 4 85 mm 85 mm 41 nH 41 nH 5 88 mm 88 mm 43 nH 43 nH6 89 mm 89 mm 44 nH 44 nH 7 89 mm 89 mm 44 nH 44 nH 8 85 mm 85 mm 41 nH41 nH 9 81 mm 81 mm 38 nH 38 nH 10 78 mm 78 mm 36 nH 36 nH 11 55 mm 55mm 22 nH 22 nH

TABLE 2 Bottom Portion Birdcage End-ring Lengths Length Inductance FrontBack Front Back 1  73 mm  73 mm 33 nH 33 nH 2 136 mm 136 mm 77 nH 76 nH3 149 mm 149 mm 87 nH 87 nH 4 135 mm 135 mm 77 nH 77 nH 5  71 mm  71 mm32 nH 32 nH

Each of the rungs 204 in the embodiment shown in FIGS. 2-5 isapproximately 547 mm long and 6.36 mm in diameter.

As can be seen in Tables 1 and 2, the dimensions between the front andback segments for this embodiments are intended to be identical.However, actual measurements may produce manufacturing or assemblyvariation differences of two or three millimeters. Referring to FIG. 3,the end ring segments listed in Tables 1 and 2 run in a clockwisedirection, with the first segment of the upper portion located behindthe left handle 212 and the last segment behind the right handle. Forthe bottom portion, the first segment is below the right handle 212 andthe last segment is below the left handle.

FIG. 6 provides the specific varying capacitance values for the LCcircuit of the embodiment of FIGS. 2-5. These capacitance values weredetermined empirically on the bench to tune the coil to the ¹²⁹Xe Larmorfrequency at 3 Tesla. Note that FIG. 6 illustrates how a birdcage coilcan be modeled as a delay line in which the inductances (rungs 1 through16) and capacitances are selected to provide one full wavelength alongthe circumference of the coil. In the present embodiment of theinvention, the capacitance values and spacing of the rungs is variedusing electromagnetic simulations to optimize B1 homogeneity. Inalternate embodiments the length of the inductive rungs is also varied.

With this embodiment, the varied circumferential spacing between therungs 204 is generally symmetrical (as discussed above, with theexemplary embodiment there can be 2-3 mm variance) between the left andright sides (looking end-on as shown in FIG. 3). The varied spacingbetween the rungs 204 is wider in the bottom portion as compared to thetop portion. The capacitances shown in FIG. 6 are chosen based upon analgorithm designed to maintain the steady speed delay about acircumference of the birdcage coil 200 based upon the inductance valuesmeasured in Tables 1 & 2.

In an embodiment, the rung spacing is a function of: the location of thebirdcage inside the RF shield, the distance of each rung to the RFshield, and the shape of the ellipse that the rung location follows. Forexample, in the section of the birdcage coil where the shape follows anellipse that is close to a cylindrical shape and the proximity of therungs is close to the RF shield, the spacing/distance between the rungsis smaller—the rung density is larger. And in the section of thebirdcage coil where the shape follows an ellipse that is stretched (moreflat) and rungs are farther away from the RF shield, thespacing/distance between the rungs is larger—the rung density issmaller.

An important aspect of certain embodiments is that dynamic detuning ofthe birdcage coil is possible using approaches well known to thoseskilled in the state of the art. Dynamic detuning is useful forpreventing inductive coupling of the birdcage coil with receive coilsplaced inside the birdcage coil. The use of receive coils that aresmaller than the body coil permits greater sensitivity to the MR signaland offers higher Signal to Noise Ratios. The use of receive coil arraysoffers the additional advantage of enabling image acquisitionacceleration. Dynamic detuning is typically performed using resonanttraps placed at various locations in the body coil. These traps aretypically activated with bias currents that turn on diodes placed in thetrap circuitry. In one embodiment, one trap is placed in the center ofeach rung of the body coil.

Development of the asymmetrical birdcage design according to the currentembodiment is now described.

Methods:

Electromagnetic field simulations were performed for three different16-rung birdcage coil designs in electromagnetic simulation, as shown inFIG. 8: Design 1—cylindrical birdcage; Design 2—symmetrical ellipticalbirdcage; and Design 3—elliptical birdcage combined with twoellipse-halves with different short axes with rungs spaced equidistant.As shown in FIG. 9, each coil design was simulated with two RF shielddiameters, 70 cm and 78 cm, at two locations, centered and adjusted tothe patient table inside the RF shield.

As shown in FIG. 7, coils were loaded with a centered torso-shapedelliptical phantom 300 (ε_(r)=77.53 conductivity 0.4 S/m) with twocylinders 302 (ε_(r)=1) inside the phantom representing adult-sizedxenon-air filled lungs.

The birdcages of FIG. 8 were excited with 35.3 MHz, 6A current sourcesplaced at the center of each rung with phase shift increments of 22.5°.Field maps of the center axial slice and B₁ ⁺ histograms for a 17 cm×25cm×25 cm VOI centered inside the phantom were used for analyticalcomparison. A preferred deviation range inside the VOI of +/−5% from themean B₁ ⁺ was chosen as the target margin for comparison. FIG. 10 showsthe B₁ ⁺ field deviation for the central axial slice with the coils ofFIG. 8 centered inside the RF shield. All of the three birdcage designscentered have a homogeneous B₁ ⁺ axial slice profile. FIGS. 11 and 12show the B₁ ⁺ volume deviation with coils centered inside the RF shield.Again, all three of the birdcage designs of FIG. 8 have a homogeneous B₁⁺ volume distribution when centered within the RF shield, where thecylindrical birdcage (Design 1) is the most efficient.

All birdcage designs for the centered shield case show ahomogeneity >83% for the target margin of +/−5% from the mean B₁ ⁺inside the VOI. Design 1 has the highest homogeneous profile at 95%, inaddition to being the most efficient of the designs. The larger shielddiameter improved VOI homogeneity for design 3. Coil mean B₁ ⁺efficiency improved by 3.3 μT for design 1, 4.8 μT for design 2, and 4.9μT for design 3.

But results changed when the birdcage designs of FIG. 8 were moved up inthe RF shield to adjust to the patient table. FIG. 13 shows the fielddistribution for the central axial slice with the coils adjusted to thepatient table. All three of the birdcage designs adjusted to the patienttable have an inhomogeneous B₁ ⁺ axial slice profile. FIGS. 14 and 15show the B₁ ⁺ volume deviation with the coils of FIG. 8 adjusted to thepatient table. All three birdcage designs of FIG. 8 adjusted to thepatient table have an inhomogeneous B₁ ⁺ volume distribution, where thecylindrical (Design 1) is the most efficient.

Designs adjusted to the table position, however, showed highlyinhomogeneous excitation profiles. In the patient table configuration,design 1 retained the highest homogeneous B₁ ⁺ profile with 30% insidethe margin for the 70 cm shield.

To improve B₁ ⁺ homogeneity inside the VOI two optimized designs, shownin FIG. 16, were analyzed using a model that adjusted the coil to thepatient table for both RF shield diameters. Optimized Design 1 is anelliptical birdcage with two ellipse-halves with different short axeswith rungs spaced wider on the bottom half and closer on the top half.Optimized Design 2 is a stretched end-ring birdcage (non-planar end ringto account for varying length rungs) combined with two ellipse-halveswith different short axes with rungs spaced wider on the bottom half andcloser on the top half.

FIG. 16 shows the field distribution for the central axial slice withthe optimized design coils of FIG. 15 adjusted to the patient table.Both of the optimized birdcage designs of FIG. 16 adjusted to thepatient table have a homogeneous B₁ ⁺ axial slice profile. FIGS. 18 and19 show the B₁ ⁺ volume deviation with the coils of FIG. 16 adjusted tothe patient table. Both of the optimized birdcage designs of FIG. 16have a very homogeneous B₁ ⁺ volume distribution. The shield sizeaffected the efficiency.

Iterative movements of rung positions and decreasing the short axis ofthe top-half ellipse improved the B₁ ⁺ homogeneity to 88% for thesmallest and largest shield. Stretching the endings and maintainingequal areas of opposing rungs improved the homogeneity for the largershield size, increasing to 93% inside the VOI. The mean B₁ ⁺ increasedby 0.2 μT-0.6 μT.

Analysis of the three common birdcage designs showed B₁ ⁺ homogeneityand efficiency is strongly dependent on the position of the birdcagewithin the RF shield and the distance between the rungs and the shield.Optimization of the rungs' position and the coil dimensions homogenizedthe field distribution significantly under loaded condition.Furthermore, end-ring stretching improved the field homogeneity,approaching similar homogeneity for the desired target margin as on atraditional centered cylindrical birdcage, at the cost of efficiency andA-P dimensions. Data analysis of commonly used birdcage designs providedthe insight in field pattern distribution under a loaded conditionincluding dielectric effects. This resulted in optimizing rung spacingand end-ring shape for homogeneous B₁ ⁺ excitation profiles across theentire thorax volume and ultimately identifying two potential designsfor construction.

Based upon the above disclosure, it can be seen that improvedhomogeneity for such non-cylindrical birdcage coils may be provided byvarying the capacitive elements and/or inductive elements of the LCdelay circuit (with our without varying the spacing between the rungs)to provide a uniform rotational velocity of the magnetic field about acircumference of the birdcage coil in various alternate embodiments. Inan embodiment, the varied capacitive elements and/or inductive elementsare symmetrical between the left side and right side of the birdcagecoil. Alternatively, or in addition, the varied capacitive/inductiveelements involves varied inductive elements. For example, the variedinductive elements involves varied dimensions of the elongated rungs,such as, varied lengths of the elongated rungs and/or varied diametersof the elongated rungs. Various dimensions of the elongated rungs may bevaried to accommodate/generate a certain inductance provided by theelongated rungs. In an embodiment, the spacing from center to centerbetween rungs remains uniform about the circumference of the birdcage,but the dimensions of the rungs are varied (such as by varying thelengths of the rungs and/or the diameters of the rungs and/or by varyingother sizes/shapes of the rungs).

Having described the inventions by reference to example embodiments, itwill be obvious that modifications can be made to such embodimentswithout departing from the scope of the invention as claimed.

The following References are incorporated by reference:

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What is claimed is:
 1. A birdcage coil for magnetic resonance imaging(MRI) system, the birdcage coil comprising: a pair of conductive endrings, each having a non-circular shape in axial cross-section; a set ofconductive, elongated rungs extending between the pair of conductive endrings in an axial direction; and means for providing a uniformrotational velocity of a magnetic field about a circumference of thebirdcage coil.
 2. The birdcage coil of claim 1, wherein the means forproviding a uniform rotational velocity of a magnetic field about acircumference of the birdcage coil includes varying at least one ofcapacitive elements or inductive elements in an LC delay circuitincorporated into the birdcage coil.
 3. The birdcage coil of claim 2,wherein the varied capacitive or inductive elements are symmetricalbetween opposing radial sides of the birdcage coil.
 4. The birdcage coilof claim 1, wherein the means for providing a uniform rotationalvelocity of a magnetic field about a circumference of the birdcage coilincludes varying axial lengths of a plurality of the set of elongatedrungs.
 5. The birdcage coil of claim 4, wherein the varied axial lengthsof the plurality of elongated rungs are symmetrical between opposingradial sides of the birdcage coil.
 6. The birdcage coil of claim 1,wherein the means for providing a uniform rotational velocity of amagnetic field about a circumference of the birdcage coil includesvarying one or more dimensions of a plurality of the set of elongatedrungs, the one or more dimensions taken from a group consisting of, alength of the elongated rungs; a diameter of the elongated rungs; and ashape of the elongated rungs.
 7. The birdcage coil of claim 1, whereinthe means for providing a uniform rotational velocity of a magneticfield about a circumference of the birdcage coil includes varyingcircumferential spacing between a plurality of the set of elongatedrungs.
 8. The birdcage coil of claim 7, wherein the variedcircumferential spacing is symmetrical between opposing radial sides ofthe birdcage coil.